Conventional electrolytes used in electrochemical devices such as batteries, capacitors and sensors are in the form of solutions or pastes to ensure ionic conductivity, but they are associated with problems such as potential damage to the devices by leakage and the necessity of a separator for immersing the electrolytes, which limits downsizing to smaller and thinner devices. In contrast, products using solid electrolytes are free from such problems and can be easily made thinner. Moreover, solid electrolytes are excellent in heat resistance and advantageous for preparation processes of batteries or the like.
Especially, batteries using polymer-based solid electrolytes have the advantage that they are more flexible than those based on inorganic materials so that they can be processed into various shapes. However, polymer solid electrodes so far proposed still have a problem of the small output current due to the low ionic conductivity. For example, proposed methods involve incorporating a specific alkali metal salt into a mixture of an epichlorohydrin-based rubber and a low molecular weight polyethylene glycol derivative to provide a polymer solid electrode (JPA HEI 2-235957) or crosslinking polyethylene glycol diacrylate by polymerization reaction (JPA SHO 62-285954), but these electrolytes are insufficient in film strength and need a support so that further improvements would be desired in the balance of film strength, ionic conductivity, adhesion to electrodes, etc.
Recently, electric double layer capacitors comprising an ionically conductive solution inserted between polarizable electrodes made from carbon materials having a large specific surface area such as activated carbon or carbon black are often used in memory backup power sources or the like. For example, JPA SHO 63-244570 discloses a capacitor using Rb2Cu3I3Cl7 with high electric conductivity as an inorganic-based solid electrolyte. “Functional Materials” February, 1989, page 33 describes a capacitor using carbon-based polarizable electrodes and an organic electrolyte. However, electric double layer capacitors using current electrolyte solutions have problems with long-term use and reliability because they are liable to leakage to the outside of the capacitors or other troubles during long-term use or abnormalities such as exposure to high voltage. Another problem of conventional inorganic-based ionically conductive materials lies in the low output voltage because of the low electrolytic voltage.
Polymer solid electrolyte layers in batteries and capacitors serve for only ion migration, so that the batteries and capacitors can be provided with smaller overall volume and higher energy density as the electrolyte layers become thinner. Batteries and capacitors using thin polymer solid electrode layers can be provided with lower electric resistance and higher output current and charging current, thereby improving the power density of the batteries. Moreover, the cycle life can be improved because corrosion by ions, especially alkali metal ions are less liable to occur. Thus, there have been demands for polymer solid electrolytes having a high ionic conductivity and a film strength as good as possible so that they can be formed into thin films. In addition, they should have a sufficient ion conductivity at low temperatures, taking into account uses in low-temperature environments such as −10° C. or less.